core: parse vgName on getVGInfo response

GetVGInfoVDSCommand:
Parse 'vgName' property and set it for each LUN.

GetDeviceListVDSCommand -> parseLunFromXmlRpc:
Set 'vgName' as storageDomainId for each LUN.

Change-Id: I4ae7e54f76c4f3f42fad5d4559478fc6df4d662b
Signed-off-by: Daniel Erez <derez@redhat.com>